beta-Thalassemia is a major public health problem in Algeria. During a survey, a family including two cases of betaO-thalassemia was studied. The family study indicated that two of the affected siblings had homozygous beta-thalassemia; there were also both normal and heterozygous siblings, and both parents had beta-thalassemia trait. In the two cases of betaO-thalassemia there was no hemoglobin A in the peripheral blood, and no beta-globin chain synthesis in whole cell incubations. Hybridization of purified complementary DNA specific for alpha- and beta-globin messenger RNAs demonstrated less than 1% mRNAbeta relative to mRNAalpha in circulating reticulocytes, and for one case in total RNA from bone marrow. There is no apparent beta- globin gene deletion as determined by hybridization in globin cDNAbeta sequence excess. Therefore the Algerian cases studied are similar in molecular pathology to some Southern Italian and Asian cases described previously, and differ from other Italian and Chinese betaO- thalassemias, in which hybridizable mRNAbeta has been demonstrated, and from deltabetaO-thalassemia, which is caused by a gene deletion.